Using a photodiode in near darkness

Artlav

Ok, would mounting a metal plate below the board do it, or there is something more delicate at work?
I can just cover the bottom of the board with tin foil, and route all the ground things to it, is that a proper solution?

In other news - i've grounded the stepper's casing across a 0.1uF capacitor, and the noise roughly halved.
It should also be noted that the noise is less with the caret away from the motor, and more up close.

sophiecentaur

@artlav
There has to be a resistive path through any device. It is made of real material and definitely not an insulator so pure logic tells you that its resistance is bound to be less than the maximum obtainable in a purpose built 1GΩ resistor. At the very most, it would be limited by the resistivity of the package of the IC but, more likely, by the resistivity of undoped silicon (?).

yungman

First of all Sophiecentaur Asked a valid question about the op-amp, way at the beginning, I asked you to look at the other post and use the op-amp the other poster chose. I assumed you followed that. What is the op-amp you are using? You need to get low bias current of 2pA or less or else you are not going to get accurate result. Right now, you are drowned by the noise and you're not going to see the problem, when you fix the noise problem, this is going to show up.


Regarding to ground plane, you connect to the signal ground. Sooner or later, you are going to have to have one with ground plane. This is very standard practice. Designing the circuit is the easy part, grounding is where the meet is, this is the hard part of engineering.

If you really don't want to redo the board( which is easy), get a small metal box and put the whole board in. Connect the board ground to the box. When you have the shield cable from the PD going into the box, make sure you ground the shield to the box. Make sure you isolate the other side of the shield(at the PD end) NOT touching any metal conductors. You don't want to connect grounds from other part of the assembly as you might create ground loop. At the output of the box, make the battery ground and the output signal ground the same. Put a toroid on this output wires. With this, you only have one ground connection....from the battery and output side.

Grounding is very important, designing circuits are the easy part. Grounding is where you earn your keep!!! If you know how to do grounding, circuits from cook books and text books usually work!!!! People complained about following the circuits from the books and it did not work.....mostly are how they build the circuits.....grounding!!!! Op-amp circuits in books and application notes work......If you can keep it from oscillating and keep the noise out. We talk about all sort of designs here, but when the rubber hits the road, it's the grounding!!!

Don't think "ground", think of this is the return path of the signal and you respect the ground just like you respect the signal. Yes, I am making a big sting about this because I spend my career pretty much specialized in grounding and noise design. All the electronics knowledge are just matter of fact, you just handle it. The grounding is where the arts of electronics. You are dealing with low frequency stuff, as frequency goes up, circuit layout ( grounding), pcb design( grounding) become 40% of the design. If I have to convey the first and most important thing from my 30 years of design experience..............RESPECT THE GROUND!!!!

Remember I asked for the picture, The moment you talk noise, I want to see your ground!!!!

Artlav

Makes sense, but i'm not sure what exactly to look for. Does the bias current define internal resistance in some way?

I use AD820, spec says input bias current is 2pA typical.
I haven't noticed you suggesting using the same one he used.
If it's lt1012, then the spec says it have ten times more bias current.

What kind of noise would that be? Without the steppers i don't get any signs of noise.

Not really an issue of not wanting to redo it, but an issue of not wasting the effort for nothing - i want to know what causes what and how it should look like, then redo it properly and permanently.

And i kind of like the idea of making the board as small as possible, putting it into a grounded box with the PD sticking out of that, and mount the whole thing onto the caret without any extra wires. But for that i need to know that i won't need to fiddle with it later.

Ok, if i understood this right.
The toroid (choke?) separates the ground "domains", so that shielding on the outside is not interfering with the shielding on the amp? Don't really understand how this is working - what is noise?
We want to avoid different parts of the "ground" side of the circuit from having different potentials?

On the attempt:
I've wrapped the amp into duct tape, then into foil, grounded the whole thing to the amp's ground and added a choke at the wires going from it to the battery.
I've also wrapped most of the wire into the foil, not letting it touch any metal parts.

That's what it looks like now:
http://orbides.1gb.ru/orbf/inside_120228.jpg

Apparently, i didn't understood it well, since now i get nonsense instead of noise.
Best described as a threshold - there is less noise, but there is either nothing or a swift climb into something if light is added. Before the "shielding" it was way more gradual.
I.e. it now responds to a very, very narrow range of currents from PD with the same output it had from a much wider range before.

What have i missed?

yungman

I really can't answer this as I don't know your circuit. Grounding really don't change your circuit, it only give you noise if you don't do it right.

Could it be your gain is too high so either it is 0 or rail? You get rid of the noise, make sure you have all the connections, make sure the +ve input of the opamp is grounded, no open circuit. Also are you sure you want to have single supply with Vee of the opamp grounded only? I always work with dual supply.

Grounding is very involved, you can write a book just on this. You can read books in Design with EMC. That's the problem with school, all books just put ground as a triangular symbol as if you put the signal there, it will magically go away.

If you build the stepper controller with non ground plane boards, you should redo it with ground plane as bad grounding will cause more emission. Put if power bypass cap like you said one cap on those circuit improve it, do it.

Artlav

Easy to do when you know what is at work...

Devil in the details?
I have a hub where all the ground wires come together, and one wire go out to the battery. Is that normal, or should the ground wire from the amp go on a more direct route?

One thing i want to clarify - i tried earlier to run the stepper from a separate power supply, and the noise remained mostly unchanged.
Is that a different kind of beast, or the same currents-going-around kind of thing?

Can't answer that.
What would having dual supply give me?
Would i have to take care to avoid negative voltage going into the ADC?
If that is related, the spec on the amp claims "True Single Supply Operation".

As a matter of fact, can you recommend any good books on the subject?

Well, i traced it back to the "shielded" cable, apparently wrapping tin foil around it does something other than just shielding.

Now i'm trying to get the things done in a cleaner fashion.

Can you define exactly what a shielded cable for this kind of connection is?
Am i to look for a two-wire cable with a mesh around them?
Would there be any undesirably large capacitance from the shielded cable?

Then, i redid the board - put the pieces closer together to remove the extra wires, and put all the ground exits onto a piece of aluminium duct tape glued to the bottom.
Below: http://orbides.1gb.ru/orbf/bottom_120229.jpg
Above: http://orbides.1gb.ru/orbf/top_120229.jpg
Does that make sense, or should i settle for a simple metal box?

yungman

I'll try to do a simple system ground drawing later on.

First I want you to try one experiment, before doing it, I want you to double check all the connection to make sure you have good solid connections and the circuit is working.

Then with stepper motor off, double check there is no noise at the output. Use a stick to touch and move the wrapped cable from the PD to amp. If there is a lot of noise just by moving the cable. You have a secondary problem. I want you to use a stick to make sure your fingers don't create a circuit path. This kind of problem is common as the shield vibrate and induce enough noise to the amp to disturb the reading.

The usual cable we use is RG59 coax cable and just use MHV connector to hook up to the amp. So the cathode just connect to the shield and the anode connect to the center connector.

Using a coax will cut a lot of the problem of noise due to the vibration of the shield. But there still can be a problem. If RG59 is not even good enough, I would strongly suggest you to consider putting the whole amp with the PD. Or else, rigid coax with solid tube as shield would be the next, or the coax with double shield. you don't want neither one.

Try that first. If it is a problem, this is a totally different issue. At the mean time, I'll try to draw the diagram.

I saw the new picture, the ground foil looks good. On the top view, I would suggest you to bend the pin 2 of the opamp up and solder the big resistor and the 100 ohm directly onto the pin. You put the connection onto the board, you can have leakage current that ruin your reading. You might not see it now, but when you clear up all the grounding and shielding problem, it might show it's ugly head. For 1G resistor, it is not go just solder onto the board. You see articles of ground guards that you are not doing. Bent pin 2 up, and solder the two resistor onto pin 2 up in the air.

One more question, Is the PD moving by the stepper motor during operation?

Artlav

Ok, i'll try it all over tomorrow (GMT+4 here).

Vibration disturbance as in physical vibration transferred to the amp, or vibration of the shield itself making electrical noise?

If the amp is sensitive to vibration then there would be other problems to consider, including for a case of putting it to the shaky place where the PD is.

It does. All the motors do is moving the PD around.
Incidentally, there is more noise when the PD is closer to the motor.

yungman

You really have to consider putting the opamp with the PD. Vibration is a big problem with these kind of amp that I never brought up so far. We had to have rigid coax to run from detector to the amplifier module because we had turbo pumps running and they vibrate less than stepper motor. This is going to be a big problem.

Before I was only suggesting about putting the amp with PD for lower induced noise, but you might have vibration problem.

Did you do the experiment with the stick? I bet you'll see noise if you touch it. It should be easy to replace the wires and foil with a coax to test if it is acceptable when motor is running. Cross your finger that it is ok. Make sure the shield and the PD is isolated from all the metal parts.

Remember about bending pin 2 up from the board. This is important as it will give you error sooner or later.

Yes, I delt with all these in my days working with transimpedance amp.

yungman

This is a very simple grounding scheme, basically I want to make sure there is no ground current from other circuits going through the coax shield of the amp module. It is important the shield and the PD body is isolated from any metal of the carrier or else you form a grounding path.



This is the most preferred way to power up the amp, through the ADC control board. if you cannot do that, a power cable with ferite toroid is absolutely necessary to break the RF ground loop.

The fact you said noise is still there even if the stepper motor is from a separate supply don't necessary mean a lot as, if you have a common ground and you are not careful on the return path, you can still have problem.

Artlav

Well, the cable idea have ultimately failed.
Haven't got around to test the touch-it-with-a-stick issue.
Any coaxial cable i can get is too inflexible for the use, and wrap-tin-foil-around solution is not worth a permanent presence.
And even in a static fixture with a coax cable i got quite some noise with motor working.

Eventually i decided to call it, and mounted the amp right where the PD is.
That have effectively eliminated 99% of detectable noise, leaving only some flicker just below the lower edge of the range, and a strip of slightly noticeable one when the caret is right on top of the motor.
Good enough for me.

The idea with soldering the big resistor and the op-amp pin above the board wasn't useless as well - i get somewhat more consistent reading now, and the sensitivity got perceptibly higher.

All in all, thank you for helping.


Some more questions:
I suppose that increasing sensitivity isn't going to go well above 2.2GΩ?
What is likely the limiting factor?

-The resistors are wire wrapped into a coil, which got some capacitance. At the moment the resistor i got seems to have enough capacitance to avoid the need of a separate capacitor in parallel.
The bigger ones are of the same build, so i'm likely going to run into unresolvable excess capacitance (=insufficient bandwidth) pretty soon.

-Other issue is thermal noise, which if i got the math right is at 10^-14A level with 2.2GΩ, and wouldn't catch me for some time yet.

-Any other considerations down there?

Separate as in separate batteries, separate controller, separate laptop. No metallic link what-so-ever.

yungman

That's very good news. We did everything to avoid vibration all the time, it just skip my mind to ask you at the beginning!!! Should have thought about it sooner to ask whether you are running the motor while taking data.

About resistor, we use Caddock:

http://caddock.com/Online_catalog/hi...esistance.html

2.2G is nothing, we use 10G!!! Caddock is not wire wound and it does not have capacitance like you described.
In production form, you can use a stand-off place very close to pin 2 and solder things on it. Make sure you clean the flux good with de-flux multiple times. Flux residue is the killer because it form surface resistance and likely to be even lower resistance than your resistor.

Even in your setup, you should clean the area around pin 2 with de-fluxer a few times. Even the dirt between pin 2 to pin 1 and pin 2 is enough to ruin the day for you. It is very important now that you got rid of the original problem. You battle in not over yet. Now you are at the point of getting back to the circuit. Believe me, things will make sense.

But one thing, when you using high value resistor like this, settling time from vibration might still be a problem. I'm afraid that you might still see some error not in form or noise, but settle to the ultimate value.

Can you do an experiment after you get the right resistor? I want you to read the data continuously and then stop the motor. Then read the data after the motor stop and compare to the last read value when the motor is running and see whether there's a big difference. This might not be an accurate test as the value you read the moment before the motor stop might take a little bit of time to settle also, but it should be close. I guess you can make it more consistent by making the light for the detector constant throughout the experiment so the output should be static when the motor is running. If you compare to reading after the motor stop, then you can see the difference in reading between the motor running and motor stop.

This might not help you as there is not much more you can do unless you damp the vibration further. You just need to understand what you are getting into and what to expect.

yungman

I forgot, if you go higher resistance, you might need a new amp. The bias current is 2pA, it will produce 4.4mV offset. The problem is when the temperature goes up, the leakage goes up a little also.

Artlav

I'm pretty certain there is no settling issues with the 2.2G - the edges are sharp and where they should be.
Might have some delay or offset, but if there is, it is constant.

However, with either 4.7G as one or 7G as two resistors there is already noticeable blurring as if it's not fast enough.
So, for the moment i'll stick with 2.2G - it's quite sensitive enough for outdoor scenes, and there is no source of low-capacitance gigaohm resistors here that won't need transcontinental shipping.

Redbelly98

No, your original posted circuit had the photodiode in the correct orientation:
Since the photo-generated current in a photodiode is in the opposite direction from what it is for a normal forward-biased diode, the current will flow from the opamp output towards the feedback resistor and then towards the photodiode: i.e. the current direction is downward at the photodiode. This is consistent with the opamp output being at a positive potential relative to the -input (current flows to the left through the feedback resistor), as it must be since you are using only a positive voltage supply to the opamp.

If the photodiode is reversed then it will try to drive the opamp output negative, which is not allowed given the power supply configuration you are using.

yungman

So it works? remember look for a better amp for 4.7G unless you can put up with the offset error I mentioned.

Hey if the resistor work for you and you have no problem buying it for the product, go for it. Remember the parts availability is very important.

BTW, the direction of the PD in your drawing is the correct way, or else I would have said so right at the beginning and you won't go this far!!!

Artlav

Ok, i seem to be have been confused by the datasheet, where the pins are named A and C, which are actually pins A and C, not Anode and Cathode...

Very much so.
Thank you for helping.

It's a hobby project, so parts availability is a matter of annoyance.